FIG. 1 illustrates a cellular radio system comprising a number of base station sub-systems (BSSs) 30 and 31. Each base station sub-system comprises a base station (BS) and a base station controller (BSC). The base stations offer a two way radio interface to a mobile station 10 as defined by specifications of the cellular radio system. Such a radio interface comprises a plurality of defined channels, the exact arrangement and usage of which is specific to each cellular radio system. An example of one prior art system is the Global System for Mobile telecommunications (GSM) system. The radio interface offered to mobile stations in cells such as cell 1 and cell 2 consists of a number of common channels and a number of dedicated channels. A cellular radio system is often referred to as a “network” although the overall network comprises the Public Land Mobile Network (PLMN) section and a gateway connection to different public networks such as Public Switched Telecommunication Networks (PSTNS) referred to as “fixed networks”.
The mobile station 10 requesting service from the network when initiating a call initiates a signalling message on an uplink radio interface channel called RACH (Random Access Channel) via a common uplink channel. The initiated access request is received at a base station from where it is forwarded to the respective BSC. From the BSS 30, 31 the access request is forwarded further to a Mobile Switching Center (MSC) 34 that is in charge of the procedures concerning call control. The MSC 34 acknowledges the access request to the mobile station via the BBS and a dedicated connection from the mobile station to the MSC can be established by assigning necessary connection resources as required between the mobile station 10 and the MSC 34. If the call is directed to another network such as a PSTN, connection is made from an external network connection support mechanism of the MSC referred to as a gateway MSC. The gateway MSC is usually integrated into the MSC 34. A call connection may then be made to national or international networks depending on the number which has been dialled.
After the mobile station 10 has been switched on, it takes a short time period until it is fully operational. The mobile terminal 10 first acquires synchronisation in a certain cell and then starts to listen to general information which is broadcast in the network in a downlink direction. A suitable cell is selected on the basis of received signal strength and additional criteria. After having selected a cell the mobile station 10 initiates a location update to the selected cell. Finally the mobile station 10 settles down in the selected cell waiting for a paging message.
Mobile stations in the form of radiotelephones, are typically battery powered in order to be portable. Therefore, power consumption is an important issue. It is desirable to provide long mobile station usage times with infrequent battery recharging intervals. A mobile station has at least two basic functional states when it is switched on: an idle mode and an active dedicated mode. When the mobile station is on, but no call is activated, answered or no data connection is established, the mobile station is in its idle mode. Thus it is able, for example, to be paged for terminating calls and terminating short messages and the user may initiate a call or other service requests. The dedicated mode of the mobile station occurs when a dedicated connection or logical link of packet radio link is seized or initiated to be reserved. The state change from idle mode to dedicated mode can be identified by the mobile station requesting resource allocation of dedicated resources of air interface connections. The mobile station will change from the dedicated mode to the idle mode when connection release has taken place. GSM systems operate according to time division multiple access (TDMA). In such a system, co-located cells use different pilot frequencies and a receiver is tuned to listen. In the idle mode a mobile station measures each frequency of neighbouring cells in order to be using the most suitable available cell. In the TDMA dedicated mode the mobile station may request and be granted a certain set of transmission circuits which may be a particular time slot or a set of time slots. The bandwidth carried by the pilot frequency of TDMA is divided in time into eight timeslots.
EP 812 119 concerns a GSM system and discloses a method for operating a cellular telephone in standby mode. The method involves the cellular telephone measuring the currently assigned channel and at least one other currently non-assigned channel. The cellular telephone detects changes in the received signal strength indicator (RSSI), in bit error rate (BER) or in similar parameter measurements, to determine whether or not it is stationary. Once the cellular telephone has determined from the error measurements that it has become stationary, it prompts either the user or the base station to confirm that this is so. The cellular telephone then terminates the measurements of at least one of the currently non-assigned channels to conserve energy, but continues monitoring the error rate to determine when it starts moving again. The method allows battery power of a cellular telephone to be saved by avoiding measurements for handoff when the cellular telephone is stationary.
The prior art also proposes mobile stations determining whether they are stationary by using an acceleration transducer to analyse motion in respect a random motion vector.
Another mobile telecommunications system is the universal mobile telecommunications system (UMTS). An UMTS system is similar in basic configuration to the system of FIG. 1 and comprises mobile stations (referred to as user equipment (UE)) connected to a mobile network.
In UMTS, the terrestrial radio network access section is referred to as UTRA. At present the access scheme in UTRA is a form of Code Division Multiple Access (CDMA) referred to as Direct-Sequence CDMA (DS-CDMA). The information which is being transmitted is spread over approximately a 5 MHz bandwidth, and so is often called Wideband CDMA (WCDMA).
UTRA has two functional modes, FDD (Frequency Division Duplex) and TDD (Time Division Duplex), for operating with paired and unpaired radio frequency bands respectively. The ability of a mobile station to operate in either FDD or TDD mode allows for efficient utilisation of the available spectrum according to the frequency allocation in different regions. At present UTRA is defined in technical specification TS 25.201 “Physical layer -General description” of the Third Generation Partnership Project (3GPP). The radio access network RAN definition of TDD is presented in technical specification TS 25.102. The FDD RAN is specified in technical specification TS 25.101.
FDD comprises a duplex method in which uplink and downlink transmissions use two different radio frequency bands. A pair of radio frequency bands, which have a specified separation, are assigned for the system. TDD comprises a duplex method in which uplink and downlink transmissions are carried over the same radio frequency by using synchronised time intervals. In TDD, time slots in a physical channel are divided into a transmission and a reception part. Information is transmitted reciprocally on uplink and downlink between the mobile station and the radio access network (RAN).
In a UTRA TDD system there is TDMA component in air interface multiple access in addition to DS-CDMA. Thus the multiple access has been also often denoted as TDMA/CDMA.
The arrangement of frames and timeslots in TDD will now be described. A 10 ms radio frame is divided into 15 slots (2560 chip/slot at a chip rate of 3.84 Mcps (Mega-chips per second)). A chip is a signal pulse cycle that is transferred by a transceiver to air interface and received by the receiver. In TDD mode a physical channel is defined as a code (or number of codes) and particular sequence of time slots.
The information rate of the channel varies with the symbol rate being derived from the 3.84 Mcps chip rate and the spreading factor. Thus the modulation symbol rates for FDD vary from 960 k symbols/s to 15 k symbols/s for uplink and from 960 k symbols to 7.5 k symbols/s for downlink. For TDD the modulation symbol rates vary from 3.84 M symbols/s to 240 k symbols/s.
Furthermore, Opportunity Driven Multiple Access (ODMA) in TDD mode can be used for relaying between nodes.
At present, channel coding and interleaving in UTRA has three optional alternatives, which are convolution coding, turbo-coding or no-channel coding. Upper layers of the network for the physical radio network indicate the channel coding selection.
In a WCDMA system, the radio network and cells have a hierarchical structure in which neighbouring cells in a cell layer use the same pilot frequency and in dedicated mode the radio link connections between the base station and the mobile station are uniquely identified by using codes to enable call identification.
A modulation arrangement will now be described with reference to FIG. 4. A receiver of a mobile station uses a correlator 61 in its RF section to despread a desired signal which then is passed through a baseband filter pass 62 in order to extract the desired signal. The desired signal is then passed on to a baseband section. Initial synchronisation of the receiver code to the RF carrier needs to be achieved before despreading may be processed.
The demodulation techniques used in receivers of mobile stations can be frequency-shift keying (FSK), phase-shift keying (PSK) or others. In WCDMA it has been agreed that quadrature phase-shift keying (QPSK) demodulation schemes will be used. QPSK occurs by using two codes (code 1 and code 2) at the same rate (shown in FIG. 5) and provides a robust radio link connection with good spectral efficiency.
In UTRA different families of spreading codes are used to spread the signal. In FDD, Gold codes are used according to technical specification TS 25.213. In TDD, either scrambling codes of length 16 or codes with a period of 16 chips are used according to technical specification TS 25.223.
Referring back to FIG. 4, in a WCDMA system, after the received signal has been despread in the correlator 61 and filtered by the baseband filter 62, the final demodulation is processed in a rake receiver which is described in the following.
Radio systems suffer from multipath propagation in which signals arrive at a receiver from a number of paths of different lengths. This leads to a summation of randomly phased signals that may cause interference. In CDMA systems having high chip rate digital signals, it is possible to obtain a benefit of multipath propagation by using time diversity. This is done by using a rake receiver.
FIG. 6 shows a mobile station used in a WCDMA system. The mobile station comprises conventional features such as an RF block 42, a deinterleaver 48, a channel decoder 49, a data buffer 81, a speech decoder 50 and a speaker 51. These features are well known and they will not be described since they are not directly relevant to the following description. The mobile station also has a rake receiver 90 located between the RF block 42 and the deinterleaver 48. The rake receiver 90 comprises a number of correlator receivers, or demodulator fingers, which typically use the same code, whose operations are displaced relative to each other by increasing time intervals of the chip interval. The rake receiver 90 also comprises a matched filter 74 which is used for filtering the output of the RF block 42.
In UMTS, each signalling or traffic channel carries its own pilot signal (a channel associated pilot signal). Because the pilot signal is not continuous, a synchronisation marker is transmitted to enable the rake receiver 90 to find the pilot signal. When the point of synchronisation occurs, output of the correlator 61 is transformed from a wideband signal into a narrow band signal of the information bearing a carrier which passes through the narrow inter frequency IF band filter (in FIG. 6 matched filter 74). Once demodulated, a control signal is generated to set the receiver's clock at its nominal rate so as to track the incoming signal. A short code with high repetition rate is used to gain synchronisation.
With a channel associated pilot signal synchronisation arrangement, the power of the pilot is lower relative to noise than would be the case for a common pilot signal. By having periodic transmission of a very short known code sequence and having a fixed time alignment with the pilot signal gives necessary pre-alignment with the pilot sequence before despreading. The occurrence of a code having good autocorrelation properties will produce a short duration spike at the output of the matched filter 74. Those spikes act as markers for time alignment with each incoming signal. Different incoming signals producing a spike may be time dispersed paths from the serving cell or neighbouring cells. The synchronisation achieved must be maintained by running the receiver's clock at a rate exactly the same as the incoming signal rate. Delay clock tracking is used having two identical codes generated by a code generator 84 in the receiver one being one chip interval behind the other. The delayed and non-delayed signals are equalised in an equaliser 83. Output of the two correlators is summed in a combiner 78.
With the channelization processed by a channel estimator 85 and a phase rotator 82, data symbols on so-called I- and Q-branches are independently multiplied with an Orthogonal Variable Spreading Factor (OVSF) code. With the scrambling operation, the resultant signals on the I- and Q-branches are further multiplied by complex-valued scrambling code, where I and Q denote real and imaginary parts, respectively.
The rake receiver also comprises a searcher 76. The searcher 76 configures the demodulator fingers to monitor specific radio frequencies and downlink channels that can be configured to receive signals from different downlink radio bearers from BSSs of the WCDMA RAN. Parallel downlink demodulation enables the mobile station to receive signals from multipaths as explained above. The searcher 76 is in charge of initial synchronisation to be accomplished in downlink. The searcher 76 also controls all procedures the receiver does concerning the mobility management procedures known as cell re-selection and handover. Cell re-selection and handover refer to the mobile station being located and served by the most suitable base station of the RAN. According to channel measurements and possibly some other criteria the searcher 76 re-configures the demodulator fingers to listen to downlink of a new base station as specified in technical specifications TS 25.304 and TS 25.305.